The acicular crystal shape of ferromagnetic chromium dioxide can be improved when the reaction mixture used for synthesis contains certain modifying agents or nucleating compounds. Numerous U.S. Patents teach this nucleating or modification technique including: antimony, U.S. Pat. No. 2,923,683; iron, U.S. Pat. No. 3,034,988; ruthenium, U.S. Pat. No. 2,885,365, alkali metals, U.S. Pat. No. 2,923,685; phosphorous, U.S. Pat. No. 3,811,942; tin, U.S. Pat. No. 2,923,684; and tellurium, U.S. Pat. No. 3,243,260. A more recent disclosure in U.S. Pat. No. 4,340,494 describes the use of fine grained antimony oxide with senarmontite crystal structure as the modifying or nucleating agent for chromium dioxide synthesis.
As a result of this prior art it is known to employ solid additions of various metals, nonmetals and oxides during chromium dioxide synthesis. Included in the solid additions to the reaction mixture in the aforementioned U.S. Pat. No. 2,923,683 are antimony metal, Sb.sub.2 O.sub.3, SbF.sub.3, SbCl.sub.2, and a mixed chromium/antimony oxide prepared by heating CrO.sub.3 and Sb.sub.2 O.sub.3 in an 8:1 molar ratio at 400.degree. C. and 750 atmosphere pressure for 3 hours.
While the mechanism of chromium dioxide synthesis and the effect of modifying or nucleating agents is not known with certainty, it is known that water included in the reaction mixture is a necessary reactant at the 100 to 700 bar pressures and 200.degree.-600.degree. C. temperatures typical for chromium dioxide synthesis. However, due to the limited solubility of the modifying agents it was impractical to add them as solutions since such an excessive amount of water would be detrimental to the synthesis reaction.
Thus, prior to the present invention the focus of effort in the field of chromium dioxide synthesis was directed towards solid additions of modifiers.
It is an object of the present invention to provide a process for manufacture of an improved modified ferromagnetic chromium dioxide for use in recording elements wherein the magnetic properties are superior to those of modified ferromagnetic chromium dioxide produced by prior art methods.